In recent years, there has been proposed a steer-by-wire type steering device for a vehicle provided with a steering wheel (handle) mechanically separated from each of wheel of the vehicle. The steering device executes control of the yaw moment of the vehicle, that is, a steering operation in correspondence with an operation applied to the steering wheel.
In the steer-by-wire type steering device for a vehicle, an electric control system plays an essential and important role in the steering function for the vehicle. Accordingly, the required level of reliability for the steering device is higher than a steering device for vehicle in which the steering wheel is not mechanically separated from each of the wheels. Therefore, in the steer-by-wire type steering device mentioned above, various measures have been conventionally taken for responding to the required improvement in the reliability of the steering device, for example, an electronic control system that is multiplexed (for example, refer to Japanese Laid-Open Patent Publication No. 2003-200840), an emergency mechanical transmission mechanism is provided (for example, refer to Japanese Laid-Open Patent Publication No. 2002-145098), or the like.
However, in the case where the electronic control system is multiplexed, the system is complicated, and manufacturing costs thereof become extremely high. Further, in the case where the steering device for vehicle is provided with an emergency mechanical transmission mechanism, an advantage of the steer-by-wire such as a high design freedom resulting from a structure in which the wheel and the steering wheel are mechanically separated, is largely inhibited. Accordingly, an adverse effect is caused in either case.